I… uh, I’m not sure? Logically, you should be able to treat it like any other language barrier but I think my brain just short-circuited trying to switch from verbal language mode to signed.
SLs are based on facial expressions/body language which is pretty universal (even if the signs themselves aren’t). In fact, now that my brain has settled into SL mode instead of VL, I reckon that speakers of two different SLs would be BETTER at navigating the barrier. They’d be more attuned to what the body and face is saying, while VL speakers rely on the words themselves.
So while using the sign-words would be fairly chaotic, the conversation itself would be happening on a level far deeper than that. I’ve said before that SL is incredibly adaptive, and I don’t think it’d take the speakers of two different SLs very long to develop their own sort of pidgin sign to communicate more freely 🙂
I believe I heard that they share about 60% of their vocabulary?
ASL and several european sign languages are heavily based on or related to FSL. Knowing one of these languages, have generally found the others to be somewhat mutually intelligible, just like knowing spanish gives you a leg up on understanding portuguese or vice versa.
However BSL is different, it is not related.
There is also IS, international sign which is a sort of pidgin or creole of all the european sign languages and ASL, and is used to communicate between them. But not everyone knows it.